A number of cathode materials for Li-ion batteries are capable of providing very high capacity when charged to voltages greater than the standard 4.2 V vs. lithium metal. For many cathode materials, the higher the charging cut-off voltage, the more lithium that can be removed from the cathode material per unit weight. Unfortunately many cathode materials in these high states of charge can be reactive towards the electrolyte, resulting in surface reactions that can damage the cathode material and consume the electrolyte. This results in accelerated capacity loss and impedance build-up, greatly shortening the life of a Li-ion cell using these materials at such high voltages. Thus it is desirable to provide high voltage capable cathode materials that are stabilized to the electrolyte at high voltages to enable the construction of long life, very high capacity, high voltage Li-ion cells. One approach to improve the stability of these materials is to coat the surface of the active cathode materials with a phase that is more stable to the electrolyte at high voltages.
Typically these materials are oxides, fluorides, or mixed oxide/fluorides of early transition metals, post-transition metals, and semi-metals (AlF3, Al2O3, etc.). Materials known as solid state Li-ion conductors (LLTO, etc.) are also used as coatings and are particularly beneficial in maintaining good C-rate capability in these devices. Introduction of a coating typically increases cell resistance through increased charge transfer resistance, electronic resistance, or both. In addition to careful selection of coating materials based on chemistry and manufacturing concerns, optimization of the amount of the coating material relative to the base material is critical in creating useful batteries with improved performance. Thus a high capacity, long cycle life cathode material that is stabilized at high voltages is highly desirable.